Elvish Term for the "Unseen"

I am trying to come up with a either a Quenya or Sindarin word for the "Unseen" or "Unseen World".

As the "Unseen" is more of a spirit world I'm not sure how certain words for land, realm, or place would fit the concept.

Q. Arda or S. gardh could be a starting point but might also describe too much of a physical realm. While Q. Ilu would probably not be a good fit to add the "invisible" qualifier to in order to refer to the unseen part of creation or everything.

The "Unseen" part itself is also not easy, one way would be to use a word for hidden or secret, or else try to take the "unseen" more literal and base something on a verb for "see" and a negation. I find the latter to make a little more sense as the "Unseen" is not explicitly a hidden world.

Something like S. Thuriardhon just doesn't have the right ring to it, if it even works. And it's maybe too close to S. garthúrian anyway which does exist in the dictionary.

Does such a term exist already or has anyone tried their hand at it or has some ideas?

In Sindarin, 'cíniel' would be the singular Active Past Participle for 'having seen'.

There is also the Sindarin prefix 'ú-', meaning 'no', 'not', 'negative', or 'impossible'. However, I am unsure how it would affect 'cíniel' when attached, as it should also undergo mutation. I believe it would change to a 'y' due to i-affection, resulting in 'ygíniel', but this doesn't seem quite right to me.

The negative form of cíniel would be either ú-gíniel or alchíniel (or perhaps ló-gíniel), depending on the negation paradigm used. But that, with "world", would mean "a world having not seen", i.e. the "world" did not see...

A passive participle would be needed here: cennen, negative úgennen / alchennen (or maybe *ló-gennen).

And not everything needs to be a compound. Compounds with negative elements don't really exist (unless the negative comes first I suppose), and I don't recall ones with participles either.

Gardh Alchennen is how I'd render it.